A radio signal from a transmitter to a receiver will travel along different propagation paths as the signal is scattered by obstacles, such as houses and other objects. This leads to signals received with different time delays, so called multipath propagation. When the receiver starts moving Doppler shifts will be introduced. When several paths arrive at the same time delay a Doppler spread, or Doppler spectrum, is generated.
Knowledge of the Doppler spread has been found to be useful for enhancing the operation of receivers as described, for example, in Morelli, M. et al., “Further Results in Carrier Frequency Estimation for Transmissions Over Flat Fading Channels”, IEEE Communications Letters vol. 2, no. 12, December 1998. The Doppler spread can also be used to infer the scalar speed of a receiver relative to local radio reflectors. This scalar speed is not the radial velocity that one can determine by measuring a Doppler shift. In most cases, the speed will correspond to the speed of the receiver, such as in the case of a receiver in a moving car surrounded by stationary street furniture.